In the field of optical communication networks, it is conventional to use an optical switch to switch paths for an optical packet transmitted from a transmission source node. A synchronization process is known for synchronizing the timing at which an optical packet is transmitted between the transmission source node and the optical switch with the timing at which the optical packet is switched.
In the following, the transmission source node and the optical switch that are used in the optical packet synchronization process will be described with reference to FIG. 10. FIG. 10 is a schematic diagram illustrating a conventional transmission source device and a conventional optical switch. As illustrated in FIG. 10, a transmission source node 1 includes a dummy packet creating circuit 1a, an optical dummy packet reading transmission circuit 1b, a time lag correction circuit 1c, and a dummy packet O/E conversion circuit 1d. 
The dummy packet creating circuit 1a in the transmission source node 1 creates a dummy packet. The optical dummy packet reading transmission circuit 1b reads the dummy packet from the dummy packet creating circuit 1a in accordance with an instruction from the time lag correction circuit 1c, performs E/O conversion on the read dummy packet, and transmits the converted dummy packet to an optical switch 2.
The dummy packet O/E conversion circuit 1d performs O/E conversion on the returned optical dummy packet received from the optical switch 2 and notifies the time lag correction circuit 1c of it. The time lag correction circuit 1c performs synchronization detection on the returned dummy packet. Accordingly, if the time lag correction circuit 1c detects synchronization loss, the time lag correction circuit 1c changes the read timing of the dummy packet and allows the optical dummy packet reading transmission circuit 1b to transmit optical dummy packets until synchronization has been achieved.
The optical switch 2 cuts out, at the optical switch timing, an optical dummy packet transmitted from the optical dummy packet reading transmission circuit 1b and returns the cut out optical dummy packet to the dummy packet O/E conversion circuit 1d. 
In the following, the transmission of the optical dummy packet and the returned optical dummy packet will be described with reference to FIG. 11. FIG. 11 is a schematic diagram illustrating a conventional time lag correction process. As illustrated in FIG. 11, when the initialization setting is performed at the time of the booting, the transmission source node 1 adds synchronization codes to both ends of the optical dummy packet and transmits the packet to the optical switch 2. The optical switch 2 cuts out the optical dummy packet at the constant optical switch timing and returns the cut out optical dummy packet to the transmission source node 1.
FIG. 11 illustrates a case, as an example, in which the head end synchronization code in the optical dummy packet returned from the optical switch 2 is cut out. In such a case, the time lag correction circuit 1c in the transmission source node 1, not detecting the head end synchronization code, lets the optical dummy packet reading transmission circuit 1b transmit the optical dummy packet by delaying the transmission timing of the optical dummy packet. The transmission timing and the size of the optical dummy packet are set after repeatedly performing this process and when synchronization is properly detected, i.e., when both ends of the synchronization codes are detected in the optical dummy packet. Then, the transmission source node 1 performs the optical packet communication by fixing the set transmission timing and size of the optical dummy packet.
Related art is disclosed in Japanese Laid-open Patent Publication No. 07-321843.
However, with the conventional technology described above, because optical packet communication is performed at the transmission timing and is performed by using the packet size that are set at the time of initial setting, there is a problem in that the transmission rate decreases. Specifically, with the conventional technology, because the optical packet communication is performed with the fixed transmission timing, the transmission is not performed at an arbitrary timing for the transmission source. Accordingly, transmission data is piled up in the transmission source node, and the transmission rate thus decreases.
Furthermore, with the conventional technology described above, because the optical packet communication is performed using a fixed packet size, if the size of data to be packetized is less than that of one packet, in order to conform the data size to the fixed packet size, the data is packetized after invalid data is added thereto. Accordingly, if the optical packet communication is performed using the fixed packet size, the data is not packetized in an arbitrary data size. Therefore, because the optical packet communication is performed using the packet having invalid data, the transmission rate decreases.